The present assignee's U.S. Pat. No. 4,084,545 describes the use of a multisolid fluidized bed as a combustor to burn various materials. This patent also discloses the recovery of heat from such combustion in a heat exchanger remote from the combustor through the use of recirculating entrained bed of fine particles. The particles pick up heat in the combustor and carry it to the external heat exchanger where the heat is given to heat exchange tubing and the heat exchange medium carried therein. This U.S. Pat. No. 4,084,545, which is incorporated herein by reference, contains a good description of apparatus which may be modified in accordance with the invention to calcine limestone in the external heat exchanger, also known herein as the secondary fluidized bed because of the addition of a fluidizing gas.
L. Reh, in his paper on "Fluid Bed Combustion in Processing, Environmental Protection and Energy Supply", presented at the International Fluidized Bed Combustion Symposium of the American Flame Research Committee, Apr. 30, 1979, in Boston, refers to well-known commercial fluidized bed calcining of alumina and suggests that limestone calcination could also be practiced in the circulating fluidized beds (p. 18). The alumina calciner consists of a circulating fluidized bed established with the combustion gases wherein an aluminum compound (aluminum trihydrate) is fed into the main combustion chamber of the fluidized bed, calcined therein, recycled internally against the gas flow and externally through a recycling cyclone and discharged in finely divided form from the bottom of the cyclone.
U.S. Pat. No. 2,700,592 discloses a method for treating endothermically-reactive, fluidizable solid particles capable of yielding an oxidizable product as a result of endothermic reaction. The endothermically-reactive particles are reacted to the oxidizable product in a first fluidized bed using heat from inert particles. The oxidizable product and the inert particles are then cycled to a second fluidized bed wherein the oxidizable product is burned to heat the inert particles for recycle back to the first fluidized bed.
The most common current methods for converting limestone to lime comprises introducing the limestone into a rotary kiln wherein it is calcined by the heat from the combustion of fuel, such as oil or gas, in the kiln. The product is a lime clinker having substantial impurities, including unburned carbon and ash, associated therewith.